Constant current sink circuit



March 11, 1969 F. E. FORD 3,432,743

CONSTANT CURRENT SINK CIRCUIT Filed Aug. 31, 1966 SOURCE INVENTOR FLOYD5 FORD I I I I I I ATTORNEY United States Patent 3,432,743 CONSTANTCURRENT SINK CIRCUIT Floyd E. Ford, Davidsonville, Md., assignor to theUnited States of America as represented by the Secretary of the NavyFiled Aug. 31, 1966, Ser. No. 576,799

US. Cl. 323--3 Int. Cl. GOSf 1/96 7 Claims ABSTRACT OF THE DISCLOSUREThe invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

The present invention relates to a constant current sink circuit andmore particularly to a constant current sink for drawing a constantcurrent discharge from a battery.

The testing of batteries frequently requires that a constant current bedrawn from the battery under test so that the total capacity of thebattery can be determined by a multiplying the current discharge leveltimes the time the discharge is maintained. One prior art method ofachieving this constant current discharge was shunting the battery witha manually operated rheostat. A constant current was maintained bymanually adjusting the rheostat to reduce the load as the terminalvoltage of the battery decreased. Obviously this method has severaldeficiencies, the most important of which are inaccuracy and therequirement of constant attention by personnel.

A second method used to discharge batteries at constant current is forcedischarging with a power supply. However, this method requires anaccurately regulated high capacity power supply with the capability offorcing a constant current from the battery terminals and dissipatingthe energy from the battery. Power supplies that have these requirementsare extremely expensive and still do not provide results as accurate asis desired.

Summary The general purpose of this invention is to provide a loadcircuit that has all of the advantages of similarly employed circuitsand has none of the above-described disadvantages. To attain this, thepresent invention provides a unique constant current sink circuit whichdoes not require an adjustment once an initial selection of dischargecurrent is made. The load circuit comprises two sections, the firstsection having a transistor and the second section a plurality ofresistors, a voltage sensing transistor and current controllingresistors, a transistor and a source of constant B+ voltage to vary theimpedance of the first section. The desired discharge current level isselected by adjusting the current controlling resistors for a coarse anda fine adjustment. The circuit then needs no further at tention untilthe battery is discharged.

It is therefore an object of the present invention to provide a constantcurrent sink circuit.

It is another object of the present invention to provide a constantcurrent sink circuit for use in battery testing.

It is still another object of the present invention to provide aconstant current battery discharge device with superior regulationcharacteristics.

It is a further object of the invention to provide a constant currentbattery discharge circuit of reduced cost.

It is a still further object of the invention to provide a constantcurrent sink circuit employing an external source of constant voltage.

:It is another object of the invention to provide a constant currentsink circuit having transistor circuitry and being adjustable forvarious discharge rates.

Other objects and many of the attendant advantages of the invention willbe readily appreciated as the same becomes better understood byreference to the following detailed description of an embodiment thereofwhen considered in connection with the accompanying drawing wherein thefigure is a schematic diagram of an embodiment of the constant currentsink circuit of the invention.

It should be noted that although the description which follows placesemphasis on the use of the device of the invention in the art of batterytesting, that the circuit of the device is useful in any applicationwhere it is desired to present a variable load to a circuit element todraw a constant current from that element.

Referring now to the drawing, numeral 10 indicates the terminal betweenwhich ground and the battery or other DC source under test is connected,and the terminal is in turn connected to the collector of NPN-typetransistor 22. The emitter of transistor 22 is connected to the seriesparallel combination of resistors 26, 32 and 34 which is in turnconnected to ground. Terminal 12 is connected to a source of constantvoltage such as a regulated B+ and through resistors 14 and, 16 and 18respectively, to the collector and base of NPN-type transistor 20. Theemitter of transistor 20 is connected to the base of transistor 22. Thecollector of NPN-t-ype transistor 24 is connected at point 17 betweenresistors 16 and 18 and the emitter thereof toground. The base oftransistor 24 is connected to terminal 30 which in turn is connected tothe sliding tap on resistor 26. Resistor 34 is also adjustable by meansof a sliding tap 36.

The circuit can be described as comprising a load having two sections,the first section being the transistor 22 and the second section thevariable resistors 26, 32 and 34, a voltage sensing means (transistor24), and current control means (including transistor 20, resistors 14,16 and 18 and the source of constant voltage) to vary the impedance ofthe first section of the load (transistor 22).

The operation of the circuit will be described in use as a constantdischarge of a battery (nominally rated at 40 volts) being discharged toa 1.5 volt level. It will be understood that the use of the device ofthe invention is not limited to such an application but in fact hasutility in any application where there is a requirement for a constantcurrent discharge from a source of voltage. The positive terminal of thebattery under test is connected between terminal 10 and ground and asource of constant regulated voltage is applied between terminal 12 andground. The desired discharge current level is selected by adjusting tap28 on resistors 26 for the coarse adjustment and tap 36 on the resistor34 for the fine current adjust-ment. The circuit then needs no furtherattention until the battery is completely discharged. During dischargethe current through resistors 16 and 18 supplies bias current to thebase of transistor 20 thereby causing it to conduct, drawing currentthrough resistor 14 from the source of constant voltage. This current isin the base circuit of transistor 22 which turns it on and causescurrent to flow from collector to emitter of transistor 22 and todevelop a voltage across the parallel series combination of resistors 26and 32 and 34. This voltage is applied to the base of transistor 24 toturn it on, causing the voltage at point 17 to decrease, and by currentdivision, decrease the current available to the base of transistor 20.Since the transistor 20 controls the base current to transistor 22, apoint of equilibrium is reached. Any change in current throughtransistor 22 produces a corresponding change in the current availableto the base of transistor 22, thereby holding that current constant. Theresistors 26, 32 and 34 work together with transistor 24 to form afeedback loop which provides a base voltage on transistor 20 that is afunction of the collector to the emitter current of transistor 22, sothat the discharge current from the battery is independent of batteryvoltage.

While the circuit has been described as employing NPN-type transistorsit is to be understood that other electronic devices capable ofcontrolling current may be used in a similar manner, such as, PNP-typetransistors and vacuum tubes.

In a typical circuit where transistor 22 is a Westinghouse 164-04transistor the circuit is capable of dissipating 200* watts whilemaintaining a ampere discharge rate with less than a 1% variation.

It will be noted that there are several alternatives to the method usedto set the discharge current. For example, fixed resistors on a gangswitch could be employed rather than the series parallel combination ofresistors shown, to set the discharge current in preselected steps.Additionally instead of employing a constant voltage source (in thiscase 28 volts) a variable source of regulated voltage could be employedto set the level of the discharge current by varying the primary bias totransistor 22.

Obviously many other modifications and variations of the presentinvention are possible in the light of the above teachings. It istherefore to be understood that within the scope of the appended claimsthe invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A constant current sink circuit for drawing a constant current from avoltage source regardless of source voltage variations comprising:

a load having first and second sections, said load being connected tosaid voltage source;

means for sensing a voltage drop across at least a part of said secondsection of said load; and

current control means comprising a transistor and an independent sourceof regulated voltage, said transistor being biased by current from saidindependent source of regulated voltage, and said transistor beingresponsive to said sensed voltage for causing variation of the impedanceof said first section of said load;

whereby current from said voltage source is maintained at asubstantially constant level.

2. The circuit of claim 1 wherein said means for sensing a voltagecomprises a transistor having a base;

said base being maintained at a potential equal to the voltage dropacross said second section of said load.

3. The circuit of claim 2 wherein said first section of said loadcomprises:

a transistor having a base, bias to said base being controlled by saidcurrent control means.

4. The circuit of claim 3 wherein the level of said constant current maybe varied.

5. The circuit of claim 4 wherein the level of said constant current maybe varied by at least one variable resistance element in said secondsection of said load.

6. The circuit of claim 4 wherein the level of said constant current maybe varied by means for varying the voltage level of said regulatedvoltage.

7. A constant current sink circuit for drawing a constant current from avoltage source regardless of variations of the voltage of said source,said circuit comprising:

a first transistor having a base, collector and emitter,

said collector being connected to said voltage source;

a series-parallel combination of resistances having a first endconnected to the emitter of said first transistor and a second endconnected to ground thereby forming a series load circuit across saidvoltage source, said series-parallel combination of resistancescomprising at least one potentiometer having a center tap;

a second transistor having a base, collector and emitter,

said base being connected to said center tap, said second transistorbeing adapted to sense a voltage drop across said series-parallelcombination of resistances and said base being maintained at a potentialequal to said voltage drop;

a source of regulated voltage connected through a resist ance to thecollector of said second transistor; and

a third transistor having a base, collector and emitter, said base beingconnected through a resistance to the collector of said secondtransistor, said emitter being connected to the base of said firsttransistor and said collector being connected to said source ofregulated voltage,

whereby said second transistor senses a voltage drop across saidseries-parallel combination of resistances, and varies the currentthrough said third transistor which is also biased by current from saidsource of regulated voltage thereby maintaining current flow throughsaid first transistor at a constant level.

References Cited UNITED STATES PATENTS 2,716,729 8/ 1955 Shockley 32372,991,407 7/1961 Murphy 323-4 3,246,233 4/ 1966 Herz 3234 3,255,402 6/1966 Vollnhals.

3,281,639 1 0/ 1966 Potter et a1. 320-39 X LEE T. HIX, Primary Examiner.

A. D. BELLINEN, Assistant Examiner.

U.S. Cl. X.R.

